“It means having access to cannabis, and ecstasy and other drugs perhaps, such as mephedrone and mushrooms - in pharmacies, possibly with electronic cards that allow you to have a certain amount per year.”

Prof Nutt said in the UK the leading causes of drug deaths were tobacco, alcohol and heroin and opiates – all much higher than deaths from ecstasy (MDMA).

“Regulated access to drugs that are less harmful than alcohol will reduce the harms of alcohol, hopefully reduce the use of drugs more harmful than alcohol like crystal meth and P and heroin, and will actually overall reduce the societal damage from drugs.”

Research on certain drugs and their potential therapeutic value was being denied, he said.

He has worked on a study of what happens in the brains of people under the influence of LSD and psilocybin (magic mushroom) using brain imaging scans.

They found “to our surprise” that the part of the brain that tends to drive depression was switched off by psilocybin, in a similar way to anti-depressants, and a follow up study suggested the effect could last for several weeks or months.

He argues psilocybin and similar drugs should bemoved from Schedule one to Schedule two - still a class of controlled drugs alongside the likes of morphine - “so that your doctors can research them very easily and determine their therapeutic value in the New Zealand population".

He says psilocybin could also be used in treatment of cancer patients and terminal illness.

"There have been two studies done ... and one of the most promising uses is in helping people come to terms with dying. Often under psychedelics people become 'at one' with the universe. I'm very interested in that, in helping people deal with that terminal, difficult part of their lives. These two American studies showed that a single psilocybin treatment helped people become less anxious about dying."

He says opiates are not the be-all and end-all in terminal pain treatment.

"Under opiates you're constipated, you're confused, you have a dry mouth - you might be free of pain but it's certainly not necessarily a good death.

"And using opiates to deal with chronic pain syndrome you can also get excessive use and people selling them on the streets so I think psychedelics offer an alternative, not just to the pain of dying but also to treat pain syndromes.

"If you allow people to take morphine for pain control, which kills in the UK nearly 2000 people a year, it's irrational to deny people access to LSD which kills nobody."

He says there are huge national variations in the types of misused and dangerous drugs, but worldwide, the big problem was going to be the synthetic opioid fentanyl, he says, because the black market has now worked out it’s much cheaper to make than heroin.

Some fentanyl is 10,000 times more potent than heroin so the tiniest drop can cause an overdose.

“It’s now become clear about a third of the opiate deaths in the United States are due to fentanyl. That is going to sweep the world”

Prof David Nutt Photo: Supplied

Prof Nutt was chair of the UK government’s advisory committee on the misuse of drugs until 2009, when he was sacked for suggesting alcohol was more dangerous than many illegal drugs.

“If we care about the harms of drugs to the point where people are dying then you’ve got to do something about alcohol.

“This is why I feel passionately about the position of science in this debate.”

Though alcohol was a leading cause of death under the age of 50 in the UK, politicians did not want to take on the alcohol industry, he says, so they make a show of being hard on drugs.

“I believe in Britain one of the reasons we have this massive problem of early death from alcohol is because we allow our kids to start drinking at the age of about 14.

“I frankly am not encouraging anyone under the age of 18 to use any drug – the longer you can wait before using any drug the less likely you are to interfere with your education or your long-term health.

“Certainly under-age cannabis use would not be good, and I encourage people not to do it. But that doesn’t mean we should ban it from people who are fit and over the age of 18."

Prof Nutt is giving a public lecture titled ‘How an enlightened approach to drug policy could revolutionise medicine as well as reducing drug harms’ at the University of Auckland on 26 February.